This is the finding of a unique study that is likely to be fiercely debated by those in favour of and those against the liberalisation of cannabis laws.

Economists Olivier Marie of Maastricht University and Ulf Zölitz of IZA Bonn examined what happened in Maastricht in 2011 when the Dutch city allowed only Dutch, German and Belgian passport-holders access to the 13 coffee shops where cannabis was sold.

The temporary restrictions were introduced because of fears that nationals from other countries, chiefly France and Luxembourg, were visiting the city simply to smoke drugs, which would tarnish its genteel image.

In a paper recently presented at the Royal Economic Society conference in Manchester they revealed that those who could no longer legally buy cannabis did better in their studies. The restrictions, the economists conclude, constrained consumption for some users, whose cognitive functioning improved as a result.

“The effects we find are large, consistent and statistically very significant,” Marie told the Observer. “For example, we estimate that students who were no longer able to buy cannabis legally were 5% more likely to pass courses.

The grade improvement this represents is about the same as having a qualified teacher and, more relevantly, similar to decreases in grades observed from reaching legal drinking age in the US.”

For low performers, there was a larger effect on grades. They had a 7.6% better chance of passing their courses. Interestingly, Marie and Zölitz found the effects were even more pronounced when it came to particular disciplines.

“The policy effect is five times larger for courses requiring numerical/mathematical skills,” the pair write.This, they argue, is not that surprising. “In line with how THC consumption affects cognitive functioning, we find that performance gains are larger for courses that require more numerical/mathematical skills,” Marie said. THC – tetrahydrocannabinol – is the active ingredient in skunk cannabis, which some studies have linked with psychosis.

The ground breaking research comes at a significant moment. The clamour for liberalisation of cannabis laws is growing.

But, as Marie and Zölitz observe in their paper: “With scarce empirical evidence on its societal impact, these policies are mainly being implemented without governments knowing about their potential impact.

“We think this newfound effect on productivity from a change in legal access to cannabis is not negligible and should be, at least in the short run, politically relevant for any societal drug legalisation and prohibition decision-making,” Marie said. “In the bigger picture, our findings also indicate that soft drug consumption behaviour is affected by their legal accessibility, which has not been causally demonstrated before.”

The research is likely to be seized upon by anti-legalisation campaigners. But Marie was at pains to say the research should simply be used to raise awareness of an often overlooked aspect of drug use: its impact on the individual’s cognitive ability. “If marijuana is legalised like it is in many states in the US, we should at least inform consumers about the negative consequences of their drug choices.”

It will also feed into the debate about THC levels in cannabis, which are becoming ever stronger. Levels of THC in marijuana sold in Maastricht’s coffee shops are around double those in the US. “Considering the massive impact on cognitive performance high levels of THC have, I think it is reasonable to at least inform young users much more on consequences of consuming such products as compared with that of having a beer or pure vodka,” Marie said. History suggests that prohibition often results in the illicit drug or alcohol trade producing ever stronger products.

Campaigners for liberalisation argue that it could help bring THC levels down and allow users to know what they are buying. The authors concede that their findings could turn out to be different if they were to replicate their study in a country that did not have restrictions on cannabis use. Marie said his work had helped inform his discussions with his teenage son. “I have a 13-year old boy and I do extensively share this with him as a precautionary measure so that he can make the best informed choice if he is faced with the decision of whether to consume cannabis or not.”